Pressure cell



M. T m m JOHN F. MCGRATH ATTORNEY J. F. MCGRATH PRESSURE CELL Filed Oct.28, 1958 n m 2 M I r/// llIlI/l/Il! Ill/Ill! 1llIl/ll/llll/l/ll/llIl/l/llllllf/Il/l/l/l/l/l/ June 21, 1960 UnitedStates Patent PRESSURE CELL John F. McGrath, Arlington, Mass, asslgnorto Baldwinljmal-fiaHamilton Corporation, a corporation of Pennsylv FiledOct. 28, 1958, Ser. No. 770,069

3 Claims. (Cl. 338-) This invention relates to fluid pressuretransducers (sometimes called cells) of the electrical strain responsivetype.

One type of such cell as heretofore proposed employs inner and outerwalls forming an annular space for fluid whose pressure is to bemeasured by electrical impedance strain gages mounted upon the walls,the strain gages responding to pressure-induced radial deformation ofthe walls. Such device is functionally deficient inits electricaloutput, frequency response, linear relation between strain andelectrical output, and hysteresis characteristics.

It is an object of my invention to provide an improved annular typefluid pressure transducer having high frequency response and electricaloutput, together with good linearity and low hysteresis.

Other objects and advantages will be more apparent to those skilled inthe art from the following description of the accompanying drawings inwhich:

Fig. 1 is a vertical sectional view of my improved transducer taken onthe line 11 of Fig. 2;

Fig. 2 is a horizontal section taken substantially on the line 2-2 ofFig. 1; and

Fig. 3 is a perspective of the inner circular wall member of thetransducer.

As shown in Fig. 1 my improved fluid pressure transducer includes a base1 having a recess 3 in which the enlarged end of an outer tubular member4 is brazed, welded, or otherwise suitably hermetically secured inposition. An inner tube 5, of preferably circular cross section andpreferably concentric to the outer tube, is provided with a relativelyheavy base 6'which has cross passages 7 therein for communication with acentral opening 8 of a threaded inlet 9. This base is welded, brazed, orotherwise suitably rigidly secured to the base 1 as indicateddiagrammatically by the dotted connecting area 10. The inner and outertubes have upper ends provided with tightly mating flanges 12 which arebrazed, welded, or otherwise suitably hermetically connected along theline 13. Bonded filament type strain gages 14 and 15 such for example asthe type shown in United States Patent No. 2,292,549 are mounted on theexposed surfaces of the tubes 4 and'5. These gages are responsive to thecircumferential deformation of the tubular walls in response to fluidpressure admitted to the annular space between the inner and outertubes. This circumferential response is preferably obtained by havingthe strain responsive resistance filaments extend in a circumferentialdirection around the walls 4 and 5. The gages may be of any desiredeffective length. As is Well understood the gage filaments are connectedby lead wires 14' to any suitable conductor extending through awell-known connector 15' which is mounted upon an outer protective shell16 secured to a base 1.

In the operation of my improved transducer fluid pressure admitted tothe annular space will cause the outer tube 4 to expand radially outwardand the inner tube 5 to contract radially inward thereby putting thegages on tube 4 in circumferential tension and the gages on tube2,942,219 Patented June 21, 1960 5 in. circumferential compression. Iaccomplish my desired object of highfrequency response and electricaloutput with good linearity and low hysteresis by reason of the innertube 5 being securely anchored at its lower end to base 1 therebyminimizing axial stresses of the tubes so that the pressure has itsmaximum effectiveness in radially deforming the tubes. As a result of myimproved cell I obtain an electrical output which is increasedappreciably more than 15% as compared to an arrangement where the innertube is not anchored at its lower end. This is because there are twoprincipal stresses acting on the walls of a pressure cell of thisgeneral type, one being axial and the other circumferential. If theaxial stress can be decreased the circumferential strain can beincreased. In prior tubular cells where the inner tube is not anchoredat its lower end of the axial force is equal to the pressure acting overthe entire projected inside area of the cell thereby resulting in thisforce placing the outside tube under axial tension and the inside tubeunder axial compression. This appreciably decreases the circumferentialstrain by reason of the operation of Poissons ratio. In my improvedcell, with the base of the inner tube 5 secured to the base 1, the areaover which pressure can act is reduced to the projected area of thesmall annular space between the inner and outer tubes. While pressureacting on this small annular area can develop a force tending to stretchthe inside and outside tubes, yet this is counteracted in my improvedcell. For instance, the axial tension stress developed in the outsidetube tends to neck it inwardly, which would slightly reduce itscircumferential expansion and thus slightly reduce the tension on theoutside gages; this eifect being balanced by the axial tension stress inthe inside tube which tends to neck it inwardly and thereby effect acircumferential contraction of the inner tube and thus increase thecompression on the inside gage. This balancing effect is reflected in anincreased electrical output of the inside and outside sets of straingages which are connected in a suitable well-known bridge to combine theoutputs from both the circumferential strains in compression andtension. In addition to this improved electrical output for a givenpressure I also have increased the frequency response of the priorannular type of cell as well as providing better linearity andminimizing hysteresis efiects.

It will, of course, be understood that various changes in details ofconstruction and arrangement of parts may be made by those skilled inthe art without departing from thefspirit of the invention as set forthin the appended claims.

Iclaim:

1. A fluid pressure device comprising, in combination, a base, inner andouter tubular members having an annular fluid pressure spacetherebetween, one end of each tubular member being directly secured tosaid base and the outer ends being hermetically connected together,whereby the inner and outer walls of the annular space are eachsubjected to tension which is induced by fluid pressure acting on thehermetically sealed end of the annular space and the inner wall beingsubjected to a circumferential compression strain while the outer wallis subjected to a circumferential tension strain upon application offluid pressure to said annular space, fluid passage means for supplyingpressure to said annular space, and electrical impedance strainresponsive means mounted on said inner and outer tubular walls so as tobe subjected to compression and tension circumferential strains therein.

2. The combination set forth in claim 1 further characterized in thatthe end of the inner tube which is secured to the base is relativelyheavy so that pressureinduced forces are not transmitted to the innertube end,

" 3."The"dotnbiflationseffofth in claim 1 further ehan References Citedin the file of this patent 'act'erizedin-thatthe 'endof theimertubewhich'is"s'e-' UNITE "ST E KTENTS cured to the base is relatively heavy sothat pressureinduced forces are not transmitted to the inner tube end,2398372 Green 1942 l utt-pelp. WW. ,l. 2,784,593 Peucker Mar. 12, 1957and fife Hind passage means cqrnpnsmg rad1a1ly extend- 5 mgf assagsinthe hezverid ftheirine'rtlibillat mem- FOREIGN PATENTS ar. t v1,065,113 France ...s--.. Dec. 30, 1953

